1. Hit ALT+F2 and type: gksu nautilus /usr/share/icons/gnome/16×16/status
2. drag the newly downloaded icons into this folder, choosing to replace the ones already there (make a backup first, mind!)
3. ALT+F2
4. type: killall gnome-panel
5. The panel will close and then restart. If it doesn’t hit ALT+F2 and type gnome-panel to get it back. Your new icons should now be visible!
5b. If the old icons remain, use: sudo gtk-update-icon-cache -f /usr/share/icons/gnome/